<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<h3>结果:<span></span></h3>

<script>
//的页面中的元素对象
let i1 = document.querySelector("#i1");
let i2 = document.querySelector("#i2");
let result_span = document.querySelector("span");

function f1() {
    //NaN: Not a Number 代表不是数, NaN和任何数值进行任何运算结果都是NaN
    if (isNaN(i1.value)||isNaN(i2.value)){
        result_span.innerText = "输入错误!";
        return;
    }
   // result_span.innerText = parseFloat(i1.value)+parseFloat(i2.value);
    //当字符串进行减乘除时会自动转成数值
    result_span.innerText = i1.value*1+i2.value*1;
}
function f2() {
    //NaN: Not a Number 代表不是数, NaN和任何数值进行任何运算结果都是NaN
    if (isNaN(i1.value)||isNaN(i2.value)){
        result_span.innerText = "输入错误!";
        return;
    }
    // result_span.innerText = parseFloat(i1.value)+parseFloat(i2.value);
    //当字符串进行减乘除时会自动转成数值
    result_span.innerText = i1.value-i2.value;
}
function f3() {
    //NaN: Not a Number 代表不是数, NaN和任何数值进行任何运算结果都是NaN
    if (isNaN(i1.value)||isNaN(i2.value)){
        result_span.innerText = "输入错误!";
        return;
    }
    // result_span.innerText = parseFloat(i1.value)+parseFloat(i2.value);
    //当字符串进行减乘除时会自动转成数值
    result_span.innerText = i1.value*i2.value;
}
function f4() {
    //NaN: Not a Number 代表不是数, NaN和任何数值进行任何运算结果都是NaN
    if (isNaN(i1.value)||isNaN(i2.value)){
        result_span.innerText = "输入错误!";
        return;
    }
    // result_span.innerText = parseFloat(i1.value)+parseFloat(i2.value);
    //当字符串进行减乘除时会自动转成数值
    result_span.innerText = i1.value/i2.value;
}


</script>
</body>
</html>